It must always be recognized and kept in mind that when cells have an alkaline electrolyte, negative electrodes as discussed above, and rechargeable manganese dioxide positive electrodes, they are assembled in their fully charged state. Accordingly, the first cycle to which any such cell is subjected in use is a discharge cycle, after which the cell is required to be recharged for subsequent use. This is, of course, in contradistinction to nickel cadmium (Ni/Cd) or nickel metal hydride (NiMeH) cells, which must first be charged before they are capable of being used. The present invention may extend to button cells, although the discussion is particularly directed to bobbin cells. In any event, any cell in keeping with the present invention is subjected to discharge when it is first put into use; and then to charge and discharge cycles. However, because of the tendency of manganese dioxide positive electrodes to swell on discharge, especially in the presence of an alkaline electrolyte, care must be taken to ensure that the positive electrode maintains its integrity and does not disintegrate or substantially swell in such a manner as to disturb the internal structure of the cell. Otherwise, the cell could be rendered ineffective for further use.
Generally, cells in keeping with the present invention have a manganese dioxide positive electrode--as discussed in greater detail hereafter--together with a negative electrode, a separator, and an alkaline electrolyte, all in a suitable container. The container is sealed by a suitable closure. In general, cells in keeping with the present invention will have a potassium hydroxide electrolyte which is 1N to 15N, and which may further have zinc oxide dissolved in it.
The separator which is used between the positive electrode and the negative electrode generally consists of an absorbent layer, which serves as an electrolyte "wick", and a barrier layer whose purposes is to prevent short circuits in the cell due to the growth of zinc dendrites which may otherwise extend between the positive electrode and the negative electrode. These properties are best obtained using a two-layer separator system, or a laminated separator.
When the negative electrode is zinc, it is generally a zinc powder mixed with a gelling agent. The gelling agent may be such as potassium polymethacrylate or polymethacrylic acid, carboxymethyl cellulose, starches, and derivatives thereof. Metallic corrosion inhibitors such as lead, cadmium, indium, gallium, bismuth, and even mercury in very small quantities, may also be included in the formulation of the negative electrode, as well as organic corrosion inhibitors, so as to reduce hydrogen gassing within the cell. Optionally, zinc oxide powder may also be included in the negative electrode formulation.
The discharge reaction of manganese dioxide is quite complex, and may proceed in various steps. A description of the manganese dioxide discharge mechanism in the presence of an alkaline solution that has been proposed by Kozawa is generally accepted, and is described in Chapter 3 of "Batteries", Volume 1, Manganese Dioxide--edited by K. Kordesch. The MnO.sub.2 discharge curve has a sloping characteristic, indicating an homogenous phase reaction. The potential of the MnO.sub.2 changes continuously while protons originating from the water of the electrolyte are introduced into the lattice of the manganese dioxide, according to the equation: EQU MnO.sub.2 +H.sub.2 O+e.sup.- =MnOOH+OH.sup.- (Equation 1).
However, the MnO.sub.2 lattice expands as an increasing number of protons are inserted into the lattice, and at a certain point during the discharge the discharge mechanism changes. After that time, the discharge may occur in a heterogenous phase reaction, according to the equation: EQU MnOOH+H.sub.2 O+e.sup.- =Mn(OH).sub.2 +OH.sup.- (Equation 2).
This second reaction step involves the dissolution of MnOOH in the form of {Mn(OH).sub.4 }.sup.-, with electrochemical reduction on the graphite additive found in the manganese dioxide positive electrode Mn(OH).sub.4.sup.=, and the precipitation of Mn(OH).sub.2 from it.
Manganese dioxide electrodes, when used as rechargeable positive electrodes in electrochemical cells, are known to be rechargeable only if the manganese dioxide is charged and discharged no more than between its nominal status of MnO.sub.2 and its fully discharged one electron status of MnOOH. For purposes of the present discussion, the theoretical discharge capacity of the MnO.sub.2 electrode between the MnO.sub.2 status and the MnOOH status is termed or designated as the theoretical one electron discharge capacity of the MnO.sub.2 electrode. If the discharge process of the MnO.sub.2 positive electrode continues beyond the MnOOH level, an irreversible phase change has been reported to occur, so that the manganese dioxide electrode is no longer fully rechargeable.
Specifically, Equation 1, above, is descriptive of the discharge reaction which takes place as the MnO.sub.2 discharges towards its MnOOH one electron discharge level in the presence of an aqueous electrolyte. Generally, the theoretical one electron discharge capacity of MnO.sub.2, as it follows the discharge reaction of equation 1, is considered to be 308 mAh/g of MnO.sub.2. It must not be overlooked that during such discharge, the structure or lattice of the MnO.sub.2 electrode expands or at least tends to expand.
Moreover, at a certain point of further discharge, the discharge mechanism may change; and after that point the discharge, which is in the second electron discharge level of the MnO.sub.2 electrode, occurs following a heterogeneous phase reaction which is set forth in Equation 2, above. Particularly with reference to alkaline manganese dioxide/zinc cells, the second step described in Equation 2, above, occurs at a voltage which is too low to contribute significantly if at all to the service life of the cell, since it occurs below 0.9 volts. Generally, it is found that with practical cells formulations, the second discharge step described above is irreversible, thereby rendering the MnO.sub.2 electrode to be non-rechargeable. Therefore, this second discharge step must be prevented from happening.
In other words, MnO.sub.2 is, in principle, capable of giving up twice its one electron recharge capacity. However, the second electron discharge capacity of MnO.sub.2, past its first electron discharge capacity, is not rechargeable in practical cells in any meaningful way, and occurs in any event at too low voltage to be useful.
With respect to prior art MnO.sub.2 /Zn cells, there have been a number of steps taken to ensure rechargeability; and specifically, steps have been taken to severely limit the discharge capacity of the negative electrode, or to provide electronic means to preclude overdischarge of the MnO.sub.2 positive electrode, so as to provide rechargeable MnO.sub.2 /Zn cells. This has been particularly of concern when it was intended to provide MnO.sub.2 /Zn cells in sufficient quantities as to make them commercially viable, meaning especially that ordinary commercially available battery grade manganese dioxide had to be relied upon.
Of course, it is generally to be noted, as well, that it is the MnO.sub.2 electrode that provides the difficulty as to rechargeability; it being generally known that it is the material of the negative electrode that is rechargeable over most if not all of the cycle life of the cell.
Historically, rechargeable alkaline MnO.sub.2 /Zn cells that have been brought to the market in the late 1960's and early 1970's were not successful because of the constraints placed upon them. Those constraints were, as noted above, the use of electronic controls to determine the end of the discharge--that is, to cut off the discharge at a certain point--or even placing the onus on the user of the cell to keep records of the amount of use that the cells were put to, and then to place the cells in the charger for recharging at an appropriate time--which must be neither too early nor too late. In general, such cells were merely modified primary alkaline MnO.sub.2 /Zn cells, and generally they had the same ratio between the active materials in the negative electrode and positive electrode as primary cells but merely employed binders such as cement to preclude structural failure of the MnO.sub.2 electrodes, as well as additives to suppress gas formation, and of course improved separators to preclude the chance of shorting between the negative electrode and positive electrode. Such cells were also quite low in respect of their energy densities: for example, a D cell may have been rated at only 2 Ah as a rechargeable cell, and it could deliver a total of only 6 Ah before the cell was completely exhausted and not further rechargeable. In such cells, the theoretical capacity of the zinc negative electrode was generally set higher than that of the theoretical one electron discharge capacity of the MnO.sub.2, at about 125% to 135% of the theoretical one electron discharge capacity. A more full discussion of the above is found in FALK and SALKIND Alkaline Storage Batteries, published by John Wiley & Sons, New York, 1969, at pages 180 to 185, and also pages 367 to 370.
Kordesch, in U.S. Pat. No. 2,962,540 describes cement bonded anodes for use in single use dry cells. The positive electrodes may have bobbin configuration, or plate configuration, and the structure of the positive electrodes is such that they are integrally united with 5% to 20% of cement additives. Optionally, a further 2% to 20% of steel wool may be employed as further cathode reinforcement. The purpose of the patent is to overcome the electrical resistance that is noted in unbonded positive electrodes, which electrical resistance is caused by the expansion of the positive electrode during discharge.
In U.S. Pat. No. 3,113,050, Kordesch describes positive electrodes that may be used in both primary and rechargeable cells. Those positive electrodes are cement and latex bonded so as to reduce expansion and contraction during discharge and charge cycles. The cement and latex binder additives are each present in the range of from 2.5% to 20%. An additional 2% to 20% of cement and/or latex binder additives can additionally be incorporated.
Alternatively, so as to overcome the limitations noted above, cells were developed by which the discharge capacity of the cell was limited by imposing negative electrode limitation on the capacity of the cell--by which it was made impossible to discharge the MnO.sub.2 to more than a predetermined amount because of the available capacity of the negative electrode. Generally, that meant that the discharge capacity of the zinc negative electrode was allowed to become no more than about 30% of the theoretical one electron discharge capacity of the MnO.sub.2 positive electrode. This, at least, preserved the rechargeable characteristics of the cell, but resulted in a cell having quite low deliverable energy capacity and density. Those limitations, understandably, mitigated against the commercial acceptability of such cells.
Reference is made to Amano et al U.S. Pat. No. 3,530,496, issued Sep. 22, 1970. Amano et al make a very strong statement of their intent to limit the depth of discharge of the MnO.sub.2 electrode by providing a negative electrode that has its capacity limited to between 20% to 30% of the theoretical one electron MnO.sub.2 discharge capacity. Amano et al prevent the mechanical failure of the positive electrode through the addition of nickel powder, which also increases the electrical conductivity of the positive electrode, and as well significantly enhances its mechanical strength. By adding nickel powder to the positive electrode, Amano et al observed a diminished tendency of the positive electrode to swell and to peel or delaminate. However, according to Amano et al, without the addition of a reinforcing agent such as the nickel powder, which acts as a binder for the positive electrode, the electrode can only be discharged to about 20% of its theoretical one electron capacity without experiencing mechanical failure of the positive electrode. As the positive electrode is only discharged to such a shallow depth, the expansion and contraction of the electrode during cycling are not extensive enough so as to cause mechanical failure. Amano et al have reported that the addition of nickel binder and graphite in a weight ratio of MnO.sub.2 :graphite:nickel of 8:1:1 increases the cycle life of the positive electrode having negatie electrodes that have capacities up to about 30% of the theoretical one electron discharge capacity of the MnO.sub.2 positive electrode. Amano et al also report that the nickel additive reduces "spring back" action of the pre-molded cathode pellets as they are released from the molding die.
How Amano et al achieve their zinc negative electrode limitations is that they provide positive electrodes having dimensions that are essentially equal to those of primary alkaline cells, and then reduce the zinc capacity of the negative electrodes by placing an annular or hollow cylindrical gelled zinc negative electrode adjacent to the MnO.sub.2 positive electrode and separated from it by a suitable two component separator. Then, the center of the negative electrode is filled with gelled electrolyte that does not have any active negative electrode material added to it. Amano et al also prefer that amalgamated copper particles be included in the negative electrode so as to enhance its conductivity. Moreover, in the negative electrode Amano et al also provide a zinc oxide reserve mass, they employ PTFE as a binder, and they must use a perforated coated screen current collector rather than a single nail which would otherwise be used in a primary MnO.sub.2 /Zn alkaline cell.
Ogawa et al, in U.S. Pat. No. 3,716,411, issued Feb. 13, 1973, teach a rechargeable alkaline manganese cell, the discharge capacity of the negative electrode of which is controlled within such a range that the positive electrode can be recharged; and wherein the negative electrode and positive electrode face each other through a gas permeable and dendrite impermeable separator. However, the Ogawa et al cell is strictly negative electrode limited in that the capacity of the negative electrode is held to be not more than about 40% of the theoretical one electron discharge capacity of the manganese dioxide. Ogawa et al discuss the fact that if a zinc-manganese dioxide cell is discharged so that its terminal voltage reaches a voltage below 0.9 volts and down to about 0.75 volts, and where the capacity of the zinc negative electrode is about the same or slightly smaller than that of the manganese dioxide positive electrode, then the effect of the discharge on the manganese dioxide is such that it is non-reversible at least in part. Ogawa et al insist that under no conditions should the depth of discharge of the negative electrode be permitted to exceed 60% of the theoretical one electron discharge capacity of the manganese dioxide positive electrode. Ogawa et al provide an alternative structure which comprises two positive electrodes, one on either side of the negative electrode, and wherein the inner positive electrode is contained within a perforated nickel plate steel pocket or canister.
It should be noted that Ogawa et al also describe an MnO.sub.2 positive electrode for secondary cells using a carbonyl nickel binder in an approach that is similar to the one described by Amano et al. Moreover, Ogawa et al utilize an unusually thick separator, having a thickness of between 0.5 and 4 mm, with the anticipation that the thick separator will provide a confinement to preclude swelling of the positive electrode in a manner similar to the metal cages used by Kordesch et al in U.S. Pat. No. 4,384,029, described below. The negative electrode employed by Ogawa et al is formed by applying a paste which contains zinc particles to a copper net or screen, which serves as the current collector. However, the negative electrode is so viscous and stiff that it must be kneaded before it is inserted into the cell. Still further, the metal screen also provides confinement for the positive electrode so as to constrain its tendency to swell during charge and discharge cycling.
Tomantschger et al, in a commonly owned U.S. patent application Ser. No. 07/893,793 filed Jun. 4, 1992, provide rechargeable alkaline manganese zinc cells that utilize an MnO.sub.2 positive electrode and a zinc negative electrode, wherein the negative electrode capacity of the zinc is limited to greater than 60% and up to 100% of the theoretical one electron discharge capacity of the MnO.sub.2. That provides a rechargeable alkaline manganese cell having higher capacity and higher energy density than has been available from the prior art cells.
What the present invention provides is cells with a manganese dioxide electrode, and which have a high capacity and a high drain capability relative to prior art cells. A negative electrode is provided, with a separator between the negative electrode and the MnO.sub.2 electrode, together with appropriate terminal means contacting the negative electrode and MnO.sub.2 electrode so as to provide respective negative and positive terminals for the cell. The manganese dioxide of the unconstrained MnO.sub.2 electrode is capable of being charged and discharged at or below the theoretical one electron discharge capacity of the MnO.sub.2 electrode, which is between the MnO.sub.2 status and the MnOOH status.
In its broadest terms, the present invention contemplates negative electrodes where the principal active component may be chosen from the group consisting of zinc, hydrogen, and metal hydrides. Other elements such as iron, lead, or cadmium might also be considered under certain conditions for special use purposes. The principal component of the aqueous electrolyte is chosen to accommodate the specific couple between the negative electrode and the positive MnO.sub.2 electrode, and particularly may be chosen from the group consisting of alkali metal hydroxides--e.g., KOH--or an acid such as H.sub.2 SO.sub.4, H.sub.3 BO.sub.3, or H.sub.3 PO.sub.4, or mixtures thereof; or a solution of salt which may be ZnCl.sub.2, NH.sub.4 Cl, or KCl, or mixtures thereof. The negative electrode is, of course, rechargeable.
In keeping with the provisions of the present invention, the theoretical discharge capacity of the negative electrode is in the range of from 60% to 120% of the theoretical one electron discharge capacity of the MnO.sub.2 electrode. In other words, the electrode balance of cells in keeping with the present invention is in the order of from 60% to 120%.
In a typical embodiment of cells according to the present invention, where the cells are intended for commercial exploitation, the active material of the negative electrode is zinc, and the electrolyte is 1N to 15N potassium hydroxide.
Cells according to the present invention may have a number of additives for purposes of enhancing the performance of the MnO.sub.2 positive electrode, or for catalyzing oxygen evolution or hydrogen recombination, or for ease of MnO.sub.2 electrode manufacturing processes, and so on. The MnO.sub.2 electrode may include at least one electrically conductive additive which is chosen from the group consisting of 5% to 15% by weight of graphite, and 0.1% to 15% by weight of carbon black. The carbon black may be present as a porous additive in the MnO.sub.2 electrode.
The addition of various barium compounds such as barium oxide, barium hydroxide, and barium sulphate in the range of from 3% to 25% may also be desired. The use of the barium compounds results in an increased cycle life and in an improved cumulative capacity of the cell.
To promote hydrogen gas recombination within the positive electrode, the electrode composition may include hydrogen recombination catalysts such as those taught in commonly owned U.S. Pat. No. 5,162,169 issued Nov. 10, 1992. The MnO.sub.2 electrode may comprise from 0.01% to 5% of a hydrogen recombination catalyst such as one chosen from the group consisting of silver, oxides of silver, silver salts, platinum, and compounds of silver and platinum.
Alternatively, the addition of a wet proofing agent such as from about 0.1% to about 3% and up to 5% of PTFE, polyethylene, or polypropylene, will enhance the gas transport within a single use or a rechargeable MnO.sub.2 positive electrode to such an extent that, as noted above, significant hydrogen recombination rates can be obtained even with uncatalyzed MnO.sub.2 electrodes. Moreover, the addition of porous additives such as acetylene black in the range of from about 0.1% to about 15%, especially if those additives have been rendered hydrophobic, also greatly enhances the gas transport capabilities of the positive electrode. It follows that the combination of a partially hydrophobic positive electrode which further employs wet proofed porous additives and which may also include a hydrogen recombination catalyst, will provide for the maximum hydrogen gas recombination rates.
Alternatively, carbon black may itself be wet proofed by treating it with PTFE. Such a product is available under the trade mark TAB-1 from IBA Shipping Center, of Torrance, Calif. The TAB-1 material is an acetylene black which has been rendered hydrophobic by the application of PTFE, and it has been developed for use in gas diffusion electrodes. Indeed, this product has been found to be well suited for maintaining gas permeability within a MnO.sub.2 primary or secondary electrode, thereby significantly enhancing the hydrogen gas permeation characteristics of the positive electrode.
Still further, so as to provide for overchange capability, an oxygen evolution catalyst as taught in commonly owned U.S. Pat. No. 4,957,827, issued Sep. 18, 1990, to Kordesch et al, may be utilized. Whatever catalyst is selected, it is chosen so as to be stable over a wide voltage range--typically from 0.75 volts versus Zn to 2.0 volts versus Zn--and also over a wide temperature range--typically from -40.degree. C. to +70.degree. C.--without any significant deterioration in performance of the cell. Such catalysts may be oxides, spinels, or perovskites of nickel, cobalt, iron, manganese, chromium, vanadium, titanium, and silver. As taught in U.S. Pat. No. 4,957,827, an oxygen evolution catalyst may be placed on the outer surface of the positive electrode, or it may be dispersed throughout the MnO.sub.2 electrode.
By adding lubricants to the positive electrode, a number of desirable effects can be achieved. In particular, the moldability of the positive electrode may be increased, and as well the amount of wear on the tools used for processing and manufacturing the positive electrode may be decreased. Moreover, it is well known that there is a "spring back" phenomenon which accompanies manufactured positive electrodes, where the positive electrode pellet will increase its size somewhat after it has been removed from the pellet press but prior to it being inserted into the cell. "Spring back" phenomenon can be significantly suppressed by using lubricants added to the positive electrode. The lubricants that have been tried are metal salts of stearic acid, or polyethylene, polypropylene, PTFE, or other polymeric materials which are otherwise benign when added to the positive electrode formulation. Such lubricant additives may be typically employed in concentrations of between 0.1% to about 3% and up to 5%, based on the weight of the positive electrode. It should be noted that such lubricant additives are becoming increasingly important in the case of low mercury or mercury free zinc MnO.sub.2 cell; and this is because any impurity which is introduced into the cell as a consequence of tool wear will generally increase the hydrogen gassing that may be experienced with the negative electrode of the cell. For example, it has been found that iron introduced into the positive electrode as a consequence of tool wear are may be fairly mobile within the cell electrolyte, so that it may eventually find its way to the zinc negative electrode of the cell. There, the presence of iron will suppress the hydrogen over-potential on zinc, and consequently it will enhance hydrogen generation within the cell.
Depending on the nature of the cell, the positive electrode may be molded into pellets and inserted into the can, followed optionally by recompaction. Otherwise, the positive electrode may be extruded directly into the can, or it may be rolled or cast as a flat electrode for use in spirally wound cells or even in respect of button or coin cells.
In any event, regardless of the specific nature of the positive electrode that are inserted into the can, it has been found that the application of a conductive carbon based coating to the inside surface of the can will provide a significant benefit in at least two respects. First, the effort required for pellet insertion or extrusion of a positive electrode into the can may be significantly reduced, and moreover, the electrical contact which is made between the can and the positive electrode is improved so that there will be a reduced internal resistance noted in the cell throughout its cycle life. Still further, an increased short circuit current for the cell will be achieved, and there will be better performance of the cell after extended periods of storage. A suitable can coating dispersion is available from Lonza Ltd. of Sins, Switzerland, under the product designation LGV 1188; and it provides a 43% aqueous dispersion of graphite and a polyvinylacetate co-polymer.
It has been well shown that a manganese dioxide positive electrode expands during discharge and contracts during charge. Kordesch et al in Electrochemica Acta 25 (1981) at 1495 to 1504, have shown that cycling an unconfined binderless manganese dioxide positive electrode resulted in electrode failure in just four discharge/charge cycles, due to its bulging and mechanical disintegration. For totally confined electrodes, 30 to 40 cycles were reported when various commercially available electrochemical manganese dioxide (EMD) were employed, and where the positive electrode was discharged only up to no more than 35% depth of discharge based on the theoretical one electron capacity of the positive electrode. Kordesch et al concluded that the failure mode was not a consequence of the build up of an insulating layer on the manganese dioxide electrode, but due to a mechanical disintegration of the electrode accompanied by a resistance increase of the electrode.
Kordesch et al also demonstrated in half cell experiments that if a similar electrode was confined by a perforated disk under pressure, the confined electrode continued its cycling life well beyond the fourth cycle; and moreover, that the change in dimension between the charged and the discharged electrode was only about half of that which occurred in the unconfined electrode. It was demonstrated that a mounting pressure of about 250 to about 750 N/cm.sup.2 was required to increase the cycle life from less than about 5 cycles--noted, above, to be because of poor conductivity and mechanical disintegration--to at least 75 cycles. A peak of 92 cycles was found at 500 N/cm.sup.2. However, it was also found that at higher mounting pressures, the cycle life would drop because of the loss of pore volume within the manganese dioxide electrode, thereby creating problems with respect to electrolyte penetration within the electrode.
When a manganese electrode is in the form of a sleeve or a disk, additional difficulties may arise. The internal resistance of the electrode may increase, and the mechanical disintegration of the electrode may be particularly severe. Kordesch, in "Batteries", Volume 1 at pages 201 to 219 discusses these problems. Several prior art references show attempts to preclude the expansion of a manganese dioxide electrode during discharge and, indeed, to try to prevent its contraction during charge. Such prior art attempts have included the addition of a binder such as cement (U.S. Pat. No. 2,962,540); the addition of graphitized textile fibres (U.S. Pat. No. 2,977,401); the addition of latex binders (U.S. Pat. No. 3,113,050); the use of combination binders such as cement and steel wool (U.S. Pat. No. 3,335,031); and the use of supplementing binders (U.S. Pat. No. 3,945,847), all as discussed above. None of those patents, however, could preclude the mechanical disintegration of the manganese dioxide electrode over many cycles, apparently due to the limited binding strength of the materials being used.
Kordesch and Gsellman in U.S. Pat. No. 4,384,029 issued May 17, 1983, teach cylindrical bobbin cells which may use mechanical enclosures such as tubes, springs, mechanical wedges, and perforated cylinders, to preclude expansion of the cathode during discharge of those bobbin cells. What that patent attempts to do is to create a constant volume manganese dioxide positive electrode, which means that the electrode must always be under a certain mounting pressure at all times. The patent suggests that by increasing the mounting pressure, the number of useable cycles for the cell will increase. By providing the metal cage, which is essentially rigid, the tendency of the manganese dioxide electrode to swell creates internal pressure within itself, which acts against the metal cage and between the cage and the can, thereby counteracting the tendency to swell; and by maintaining the manganese dioxide electrode under pressure, the electrode retains a substantially constant volume during discharge as well as charge.
A different approach, using combinations of binders with a mechanical retainer of multiple mechanical retainers is disclosed in a further patent which is commonly owned herewith, being U.S. Pat. No. 4,957,827 issued Sep. 18, 1990 in the names of Kordesch, Gsellman and Tomantschger.
While the two Kordesch et al patents noted immediately above show the use of means such as cages to accomplish rechargeable cells having cycle lives of up to several hundred cycles, there are also several disadvantages from the approaches taken in the two Kordesch et al patents that must be considered. In particular, where cement or other non-conductive binders are used, they are present in the range of typically 5% to 10%, or even up to about 20%, by volume of the manganese dioxide electrode, and therefore the quantity of active ingredient that can be placed in the electrode is reduced. This results, of course, in a decrease in the useable discharge capacity of the cell, and it may also result in a decrease in the conductivity of the manganese dioxide electrode. On the other hand, if an insufficient amount of binder is used, then typically the manganese dioxide electrode may tend to crumble and/or crack, so that a coherent electrode structure is not achieved and its integrity is seriously affected.
If mechanical structures such as cages or screens are employed, then there is a significant increase in the material cost of the cell, as well as a significant increase in the cost of assembly of the cell. Indeed, there may be a significant effect and complication with respect to the use of high speed production equipment. Moreover, the use of a mechanical component such as a perforated iron or copper cage or plate may significantly increase the probability of cell gassing within the cell.
Still further, the use of a mechanical cage of screen adjacent to the separator of the cell may significantly affect the capability of the cell to operate in high drain conditions. Any mechanical means which restricts the electrode interface between the positive electrode and the negative electrode will act to limit the current density achievable from within the cell.
In contradistinction to the prior art, which relied upon the use of mechanical structures such as cages or screens, or the use of binders such as cement and steel wool, commonly owned U.S. Pat. No. 5,108,852 issued Apr. 28, 1992 to Tomantschger and Michalowski describes the use of an unconstrained manganese dioxide electrode for use in rechargeable galvanic cells. There is no cage or screen, and the patent is directed to the manner in which the positive electrode is constrained from significantly changing its dimensions by essentially filling the entire space allotted for it within the cell, while permitting perhaps a slight accommodation for height-wise or longitudinal expansion or growth in bobbin cells, or crosswise expansion or growth in button cells. Certain additives are contemplated for use in the manganese dioxide electrode mix, including conductive fibres or graphite and optionally including metal-based additives.